It is accepted that two of the most important challenges facing semiconductor designers are to reduce the power consumption of devices and to increase their manufacturability. There is an ever-growing number of applications where minimizing power consumption is of critical importance, e.g. space electronics, implants, wireless sensor networks, wearable electronics, mobile phones, laptops and PDAs to name but a few. However even in applications where an integral power source is not necessarily required, such as desktop personal computers, the impact of Moore's law (according to which microprocessors tend to double the number of transistors every 2 years) means that power densities are making it increasingly difficult to provide effective cooling. This is hampering efforts to increase miniaturization. The increasing densities are also making processors more prone to manufacturing defects.
Moore's law cannot continue if the power consumption associated with existing CMOS circuits is maintained, as was demonstrated by Gelsinger in Microprocessors for the new Millennium: Challenges, Opportunities and New Frontiers, Proc IEEE International Solid State Circuits Conference, 2001, pp. 22-25 where it is illustrated that predicted powers are excessive and prohibitively large for any practical application. For example, a simple extrapolation of current trends suggests power densities of the order of those found in nuclear reactors and rocket nozzles before the end of this decade. It is clear that total power consumption will become the limiting factor for future CMOS circuits.
The increasing density of components in microprocessors is not only problematic from the point of view of reduced spatial area for each transistor in which the heat generated by current flowing through it must be dissipated; but also from the point of view that as layers become thinner there is a greater average leakage tendency from the transistor gates as a result of quantum tunneling. This increases the standing current consumption of processors and therefore their average operating temperatures.